The silaguanidinium cation and the search for a stable silylium cation in condensed phases

Quantum mechanical calculations at the MP 2/6-31 G(d) level are reported for the silaguanidinium cation Si(NH₂)₃⁺ (1) and derivatives thereof. The equilibrium structure 1a has D₃ symmetry with planar amino groups rotated out of the SiN₃ plane by 19.6°. The Si-N bond length of 1a (1.658 Å) is intermediate between a single and a double bond. Isodesmic reactions show that the stabilization of the silylium cation 1a by the amino groups (63.5 kcal mol^-1) is about 40% of the resonance stabilization of the guanidinium cation (159.3 kcal mol^-1), but 1a is clearly better stabilized than alkyl-substituted silylium cations. The electronic stabilization of 1a by the amino groups is also made obvious by the calculated complexation energy with one molecule of water. The calculated stabilization through complexation of water at HF/6-31G(d) is markedly lower for Si(NH₂)₃-(H₂O)⁺ (6) (28.8 kcal mol^-1) than for SiMe₃(H₂O)⁺ (40.6 kcal mol^-1). The tris(dimethylamino) silylium cation Si(N-Me ₂)₃⁺ (8) is even more stable than 1a. The complexation energy of Si(NMe₂)₃-(H₂O) ⁺ (10) is only 17.3 kcal mol^-1. IGLO calculations of the ²⁹Si NMR chemical shifts predict that 1a and 8 should not show the same extremely low shielding that is calculated for alkyl-substituted silylium ions. The calculated ²⁹Si resonances for 8 are in reasonable agreement with the experimental NMR spectrum of (Me₂N) ₃SiB(C₆F₅)₄. AM1 calculations predict that the substituted tripyrrolidino silylium cation 12 would be an even better candidate for a stable tricoordinate silylium cation in condensed phases. One of the pyrrolidine rings of 12 has tert-butyl groups in the 2 and 5 positions, which serve as a steric fence around the silicon atom.